Orthoceras

This belemnite-like cephalopod was actually more closely related to the present-day Nautilus. This fossil from Morocco has been polished to show the compartments that the cone-shells were divided into. Each compartment or camera is divided from the next by a wall or septa but connected by a central tube called a siphuncle.
After the death of the occupant the compartments filled with gases as the soft parts decayed and, in some cases, kept the shells afloat. Fossils show that they would drift along broad end first, sometimes trailing the pointed end along the sea bed.
Just as cockle shells tend to get concentrated in a particular part of a sandy beach, orthoceras shells are sometimes found sifted into such concentrations that the fossils are called ‘Othoceras’ limestones.
Orthoceras was a predator and scavenger of the Ordovician and Silurian, 400 to 500 million years ago. It swam with it shell horizontal in the water.